drm/vmwgfx: Reserve fence slots on buffer objects in cotables

The buffer objects created by cotables were missing fence reservations.
They are created from vmw_validation_res_validate which makes them miss
the ttm_eu_reserve_buffers which is called from vmw_validation_bo_reserve.

Cotables are the only resources which create a buffer object in the
create callback so make sure the code also reserves the slots.

Signed-off-by: Zack Rusin <zackr@vmware.com>
Co-developed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Fixes: c8d4c18bfbc4 ("dma-buf/drivers: make reserving a shared slot mandatory v4")
Link: https://patchwork.freedesktop.org/patch/msgid/20220422161342.1142584-1-zack@kde.org